The present invention concerns a procedure for boosting the ventilation in paper machine drying sections with closed hood, the procedure involving blowing onto surfaces suspectible to condensate precipitation, air which is in an appropriate state.
In a modern paper machine, the primary heat consumption is almost entirely incurred in the drying section. The specific primary thermal consumption in the drying process per unit of water evaporated is dependent on the humidity of the exhaust air.
Endeavours have all the time been directed to increasing the humidity of the exhaust air from the paper machine hood. In the case of open hoods, the humidity of the exhaust air was 50-80 g per kg of dry air. With closed hoods the value of 100 g per kg of dry air was reached in the 1960's, and this was brought up to 120 g per kg of dry air in the 1970's. The dimensioning value used for the exhaust air has been 130-140 g per kg of dry air in recent years.
The obstacle to raising the humidity past 150 g per kg dry air is precipitation of so-called condensate, or condensation of water vapour, on the hood structures. This phenomenon occurs when the surface temperature of the structure is lower than the dew-point of the air in respective state. Condensation is a highly detrimental phenomenon in the hood for the reasons, among others, of accelerated corrosion of the hood structures and of water drops falling on the paper web.
Various measures have been used in attempts to prevent occurrence of condensation in the hood at high exhaust air humidity. Of such may above all be mentioned improved thermal lagging of the hood. While still in the 1960's the thermal lagging had a thickness about 50 mm, nowadays quite commonly a thickness about 100 mm is already employed. Another measure used towards preventing condensation is to blow dry and hot air within the hood at the points where condensation occurs. This procedure has been disclosed in a paper by the company Svenska Flaktfabriken Ab entitled "Modern Paper Machine Hoods and Pocket Ventilation Systems" delivered at the felt symposium arranged in Halmstad, in July 1981, by Nordiskafilt AB.
However, if it is desired to increase the humidity of the exhaust air from the hood considerably beyond the present, e.g. up to 200 g per kg dry air, condensation can no longer be avoided by increasing the thickness of the hood's thermal lagging.
This is due to frequently high local humidities or to locally low surface temperatures occurring at so-called thermal bridges and points of leakage. The doors and windows of the hood are particularly concerned in this respect. Elimination of thermal bridges and leaks would require such expensive solutions that these cannot be carried out in practice.
Blowing of dry and hot air at the points susceptible to condensation also fails to solve the problems arising from condensation because there simply is not enough air to this purpose, all the air having to be used to increase the evaporation. This is particularly the case when using high exhaust air humidity, because as the humidity level of the air within the hood increases, evaporation is impeded with diminution of the humidity gradient between the surrounding air and the saturated boundary layer upon the evaporative surface, i.e., the web. Moreover, the reduced exhaust air quantity even in itself results in a reduced replacement air quantity, since the replacement air accounts for only about 65-80% of the exhaust air.
In the U.S. Pat. No. 4,268,974 is disclosed a paper machine hood where the frame structure of the hood also serves as a system of air passages. The heat recovery has also been accommodated within the hood. Although in the hood according to U.S. Pat. No. 4,268,974 the temperature can be made so high that no condensation occurs, the condensation of water vapour e.g. at windows and doors still remains unsolved.